Cut Christmas trees must be watered and kept hydrated to last through the holidays and to prevent them from becoming fire hazards. This includes small tabletop trees.
Many trees are shipped from as far as Oregon. Buying early will give you better selection and a fresher tree. Before buying, run your hands through the needles to check for suppleness. Hold the tree upright and drop it from a few inches. Excessive needle drop is a sign of dryness.
Before installing, you will need to remove some of the lowest branches and trim an inch off the bottom of the trunk. Make a square cut, not an angled one. Tree lot merchants will do this for you, but it is better to do it at home. You will need gloves, a bow saw or chain saw for the trunk, and lopping shears to remove the branches.
It's OK to leave the tree in a bucket of water for several days until you have time to install and decorate it — but monitor the water level. If you are storing it in, say, an unheated porch or garage, make sure it won't tip or blow over. The tree is thirstiest when first cut and will absorb several quarts of water in its first day or two. Keep a close eye on the water level and don't let it drop below the cut, which will gum up if it dries.
When taking it indoors, make a fresh cut. Place the tree away from fireplaces and heat registers and away from doors. Make sure your tree stand is big enough for the tree, for both stability and the amount of water storage. Additives to the water are not necessary, but the level should be checked daily and topped off as needed — use your finger to check the water level.
Additional tips can be found at the website of the National Christmas Tree Association, Realchristmastrees.org.